1. Field of the Invention
The present invention relates to a communication system and a transmission station which can retransmit data in which a transmission error has occurred, and in particular relates to a communication system and a transmission station which can finish transmission as soon as possible even when a transmission error has occurred, and can decrease traffic in the retransmission of the data.
2. Background
In conventional communication systems which transmit data bidirectionally, the receiving side requests retransmission of data by transmitting a data retransmission request signal to the transmitting side when an error in the data signal is detected. When the receiving side receives a data signal correctly, it provides notification that the data signal was received correctly by transmitting an acknowledgment signal to the transmitting side. When the transmitting side receives a data retransmission request signal, it retransmits the data signal. The transmitting side then transmits the next data signal after it receives an acknowledgment signal from the receiving side.
When the transmission delay in a line is small, then the time until the transmitting side receives the acknowledgment signal or the retransmission request signal is short. Thus, the transmitting side can immediately transmit the next data or the data for retransmission. However, when the transmission delay in the line is large, then the time until the transmitting side receives the acknowledgment signal or the retransmission request signal is long. As a result, the next data or the data for retransmission cannot be sent immediately, causing the rate of utilization of the line to deteriorate.
The transmitting side carries out outstanding transmission in which it transmits data a predetermined number of data signal frames, without receiving an acknowledgment signal or a retransmission request signal from the receiving side. The number of data frames which are transmitted in succession without receiving an acknowledgment signal is referred to as the "outstanding number."
In outstanding transmission, the acknowledgment signal has an identification number for identifying a data signal frame for which receipt has been confirmed. The retransmission request signal has an identification number for identifying a data signal frame for which retransmission has been requested. The transmitting side recognizes which data signal frame has been correctly transmitted by using the identification number of the acknowledgment signal. The transmitting side also recognizes a data signal frame to be retransmitted on the basis of the identification number of the retransmission request signal.
Among the methods for retransmitting data in outstanding transmission, there are available methods such as the GO BACK N method, which retransmits the data requested for retransmission and all data thereafter, and the SELECTIVE REJECT retransmission method, which retransmits only the data which is requested for retransmission.
FIG. 9 is a sequence diagram showing the data retransmission sequence in the GO BACK N method. The numbers in the brackets show each value of the identification number of the data signal. The data signals having from 11 to 15 as the identification number, that is, from data signal (11) to data signal (15), are transmitted continuously from station 10 to station 20. When an error occurs in data signal (12), station 20 transmits retransmission signal (12) by using identification number 12. Station 10 retransmits all data on and after the data (12) requested for retransmission. In other words, station 10 retransmits the data from data signal (12) to data signal (15).
FIG. 10 is a sequence diagram showing the data retransmission sequence in the SELECTIVE REJECT retransmission method. When station 20 transmits selective retransmission request signal (12), station 10 retransmits data signal (12) only. In the GO BACK N method, data signals transmitted correctly are also retransmitted. However, in the SELECTIVE REJECT retransmission method, only the data signal in which an error has occurred is transrnitted. Therefore, in the SELECTIVE REJECT retransmission method more data per unit time can be received than in the GO BACK N method.
However, in the both GO BACK N method and the SELECTIVE REJECT retransmission method, the traffic of the line increases due to the fact that correct data are retransmitted along with the data in which an error actually occurred. In the outstanding method, all correct data are retransmitted so that the traffic of the line increases still further. In particular, in the case where the quality of the line is poor, making it highly likely that retransmission will be necessary, the transmitting side must repeatedly transmit the same data signal. As a result, the time until the transmission of data is complete becomes longer. Further, in case where the quality of the line is poor, the transmitting side cannot transmit data and the line might be disconnected.
In general, the length of the data frame is inversely proportional to the probability of data retransmission. Therefore, in the case where the quality of the line is poor, it is possible to decrease the probability of data retransmission by making the data frame shorter. In this case, however, the transmitting side must transmit more data frames, so that the load on not only the transmitting side and the receiving side, but also on the switching apparatus present on the line increases. Also, the data transmission speed decreases because the receiving side must interpret the header for each data frame.